1. Field of the Invention
This invention relates to a radiation image read-out apparatus wherein an image signal is obtained by reading out a radiation image of an object from a recording medium, such as a stimulable phosphor sheet or photographic film, on which the radiation image has been recorded, and image processing is carried out on the image signal. This invention also relates to a method for operating the radiation image read-out apparatus.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. Novel radiation image recording and reproducing systems which accurately detect an image signal have been proposed. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order approximately to ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term "read-out conditions" may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed in the main scanning direction or in the sub-scanning direction.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary readout image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
In the course of adjusting the read-out conditions for the final readout and/or the image processing conditions, various modes are often used. For example, a mode is used in which a histogram (or a probability density function) of the image signal (or the preliminary read-out image signal) is created, and the read-out conditions for the final readout and/or the image processing conditions are determined automatically from the histogram each time the image signal is detected. In another mode, a mean-level value, such as the mean value, the median value, or the like of the image signal (or the preliminary read-out image signal) corresponding to the whole are or a partial region on the recording medium is calculated, and the read-out conditions for the final readout and/or the image processing conditions are determined from the mean-level value. (The mean-level value will hereinafter be often referred to as the "mean value". This mode will hereinafter be referred to as the "mean value operating mode". Also, the region for which the mean value is calculated will hereinafter be referred to as the mean value operating region".) In a further mode, the readout conditions for the final readout and/or the image processing conditions are determined and stored in advance for various characteristics of recorded images. Such characteristics include, for example, what portion of an object is represented by the recorded image (e.g., the head, the chest, or the abdomen in cases where the object is a human body) and what mode was used when the image was recorded (e.g., an ordinary image recording mode, a contrasted image recording mode, or an enlarged image recording mode). A mode is also used in which factors important in determining the read out conditions for the final readout and/or the image processing conditions are designated manually each time the conditions are to be determined. However, every mode of adjusting the read-out conditions for the final readout and/or the image processing conditions has advantages and drawbacks. Therefore, various modes are used in combination such that their drawbacks can be compensated for.
The aforesaid mean value operating mode includes a mode of selecting a region from a plurality of mean value operating regions. The region selecting mode will hereinbelow be described in detail.
FIG. 3 is an explanatory view showing an object image (radiation image) stored on a stimulable phosphor sheet and a plurality of (in this example, nine) mean value operating regions which are set in the mean value operating mode.
With reference to FIG. 3, an object image 21a (radiation image) of the head of a human body is stored at approximately the center portion of a stimulable phosphor sheet 20. Therefore, in this case, a mean value operating region 22a, which is located at the center among a plurality of mean value operating regions 22, 22, . . . is selected. A calculation is made to find the mean value Sa of the image signal (or the preliminary read-out image signal) corresponding to the mean value operating region 22a. Thereafter, information about a desired reproduced image density D0 and information about a desired image signal range L are entered manually into the radiation image read-out apparatus. The information about the desired reproduced image density D0 designates which level of image density in a reproduced visible image the mean value Sa should have. The information about the desired image signal range L designates which range of the image signal including the mean value Sa is to be used in the reproduction of a visible image (i.e. which contrast the reproduced visible image should have). The read-out conditions for the final readout and/or the image processing conditions are determined from the mean value Sa, the desired reproduced image density D0, and the desired image signal range L. The final readout is carried out under the determined read-out conditions, and/or the image processing is carried out under the determined image processing conditions. Thereafter, a visible image having the desired image density and contrast is reproduced.
FIG. 6 is an explanatory graph showing the relationship between the mean value Sa, the desired reproduced image density D0, and the desired image signal range L. This graph is for the case where a preliminary readout is carried out. The values of the preliminary read-out image signal SP, which were obtained during the preliminary readout, are plotted on the horizontal axis at the right part of the graph. The frequency of occurrence of the values of the preliminary read-out image signal SP, which were detected from, for example, the whole surface of the stimulable phosphor sheet 20 shown in FIG. 3, is plotted on the vertical axis at the upper part of the graph. The values of the image signal SQ obtained during the final readout are plotted on the vertical axis at the lower part of the graph. The image density level D of a visible image reproduced from the image signal SQ is plotted o the horizontal axis at the left part of the graph.
In FIG. 6, a probability density function 10 represents the frequency of occurrence of the values of the preliminary read-out image signal SP, which were detected from the whole surface of the stimulable phosphor sheet 20. The probability density function 10 ranges from the maximum value Smax to the minimum value Smin. Also, the mean value Sa of the preliminary readout image signal SP corresponding to the region inside of the mean value operating region 22a shown in FIG. 3 corresponds to the level S0 shown in FIG. 6. In such cases, when a desired image signal range L, which defines the image signal range from S1 to S2, including S0, is designated, the read-out conditions for the final readout are set so that during the final readout the minimum image signal value Qmin and the maximum image signal value Qmax are obtained respectively from the picture elements represented by the values S1 and S2 of the preliminary read-out image signal SP. Specifically, the read-out conditions for the final readout are set so that during the final readout the image information represented by values of the emitted light signal falling within the range of S1 to S2 is detected as an image signal having values lying on the straight line G1 shown in FIG. 6. From the image signal SQ obtained under the read-out conditions for the final readout, a visible image is reproduced such that the image signal value Q0 corresponding to the value S0 of the preliminary read-out image signal SP yields a desired reproduced image density D0 in the reproduced visible image and the minimum image signal value Qmin and the maximum image signal value Qmax of the image signal SQ correspond respectively to the minimum image density Dmin and the maximum image density Dmax within the predetermined correct image density range of the reproduced visible image. Specifically, a visible image is reproduced such that the values of the image signal SQ falling within the range of Qmin to Qmax correspond to the image density levels lying on the straight line G2 shown in FIG. 6. As described above, in the mean value operating mode (region selecting mode), the mean value S0 is calculated. Also, as the desired image signal range L and the desired reproduced image density D0 are designated, a visible image having appropriated image density and contrast can be reproduced.
In the radiation image read-out apparatus wherein various modes, including the region selecting mode, are used as the modes of adjusting the read-out conditions for the final readout and/or the image processing conditions, when the region selecting mode is to be used, information designating the region selecting mode and information about the mean value operating region, which is to be selected from among a plurality of mean value operating regions (i.e. information about at which part of the recording medium an object image is present), are entered by an operator into the radiation image read-out apparatus. Also, the desired reproduced image density D0 and the desired image signal range L are designated by the operator. After these items of information are entered into the radiation image read-out apparatus, the radiation image is read out from the recording medium.
However, it often occurs that the position on the recording medium at which an object image (radiation image) is actually recorded deviates from a position intended during the recording of the radiation image. For example, it often occurs that, even though an operator carried out a radiation image recording operation with the intention of recording an object image (radiation image) at the center portion of a recording medium, the object image is actually recorded at a portion of the recording medium outside of its center portion. By way of example, as shown in FIG. 3, it often occurs that, in cases where an object image is actually recorded at the position indicated by the solid line 21b, the operator considers by mistake that the object image 21a is recorded at the center portion of the stimulable phosphor sheet 20 and designates the mean value operating region 22a as the region for which the mean value of the image signal is to be calculated. In such cases, the mean value Sa of the image signal (or the preliminary read-out image signal) corresponding to the designated mean value operating region 22a is calculated. However, the calculated mean value Sa will often be markedly different from the correct mean value of the image signal representing the object image. Because the image density level of a visible image reproduced from the image signal is set based on the incorrect mean value Sa, a reproduced visible image is obtain which has an inappropriate image density, for example, a very low image density.